The present invention relates to a delivery vehicle for dispensing therapeutic drugs and other medications in the eye. More particularly, the invention relates to a delivery vehicle which may be inserted in the eye for dispensing medication in the eye at a controlled rate over an extended period of time.
Treatment of both temporary and more permanent disorders of the eye has long been accomplished by placing liquids, salves, or other medicaments directly in the eye by means of a squeeze tube or an eye dropper or by flushing the eye with a wash glass. More recently, however, delivery vehicles have been developed in which medication is confined within a small semipermeable body which is inserted under one of the eyelids. The medication is diffused from the body into the tear or lacrimal fluids in the eye and is dispersed over portions of the eye and surrounding areas by the lacrimal fluids.
One form of delivery vehicle inserted into the eye is known as a "soft contact lens". Soft contact lenses consist of a highly porous plastic which can absorb water or other fluids up to 55 per cent of its volume. By soaking such a lens in a fluid medication and inserting the lens in the eye in conventional fashion, a dosage of the medication is distributed in the eye by the lacrimal fluids.
Certain difficulties arise with treatment by means of soft contact lenses. While they absorb a substantial quantity of medication, the plastic material of which they are comprised cannot control the release rate of the medication and as a consequence, strong doses of the medication are dispensed in the eye initially and weak doses after a protracted period. Such variations in the dispensing rate resemble the treatment obtained by periodic applications of medication from an eye dropper and is not the most effective method of treating chronic or more persistent disorders such as glaucoma. Initial doses are strong and lose their effectiveness after a certain period of time. The effects of the medication can be extended by increasing the initial concentration of drugs utilized in the medication; however, the danger of irritation from high toxicity limits such an approach. Additionally, a soft contact lens, as any contact lens, blocks the supply of oxygen to the cornea and, hence, continuous use of the lens for periods of more than twenty four hours is not recommended. Interrupting treatment by removing the lens for rest periods aggravates the difficulty of providing a constant level of medication to the eye.
Another type of delivery vehicle which has overcome many of the difficulties associated with soft contact lenses employs a body of polymeric plastic in which a reservoir of medication is held. The polymeric material can be designed to control the release rate of the medication and thus provide a more uniform level of medication within the eye for extended periods of time. U.S. Pat. Nos. 3,618,604 and 3,828,777 issued to Richard A. Ness disclose delivery vehicles of this type in detail. In practice, the body has a small ellipsoid or bean shape and is inserted in the conjunctival sac between the sclera and one of the eyelids. The medication diffuses through the polymeric material to the surface of the device and is spread over the surface of the eyeball by the lacrimal fluids. The great benefit obtained by these devices is the controlled rate at which the medication is released from the device by the polymeric material. The medication reaches the eye at a relatively uniform rate compared with soft contact lenses and the supply of medication in the device is not immediately expended but, instead, is dispensed gradually over a protracted period of time.
One difficulty which arises from all types of devices inserted in the eye is the sensitivity of the eye to foreign bodies which make contact with the eye, especially in the corneal region. A foreign body can become a source of irritation to the patient which renders the use of an insert impractical and offensive unless it is held in an area of low sensitivity. Irritation stimulates defense mechanisms which protect the eye against foreign objects. The lacrimal glands become more active and produce tearing to wash the object from the eye. Even if tearing is unsuccessful in dislodging an insert, the tear fluids act as a diluent to the dispensed medication and also leach further medication from the delivery vehicle. Excess tearing results in overflow of the conjunctival sacs and, as a result, the medication itself is washed out of the eye in tears.
Another reaction of the eye which aggravates the retention problem is the increased eye mobility caused by local irritation. The eyeball tends to move up and down while the lids open and close in order to expel a foreign object. Even if an insert is not expelled from the eye by these reactions, it can be dislodged to a point where it contacts a highly sensitive area, such as the cornea. In most patients, contact with the cornea cannot be tolerated even momentarily and the eye defense mechanisms are brought immediately to a highly activate state which usually results in expulsion.
It is, accordingly, an object of the present invention to provide an ophthalmic delivery vehicle of the type which provides a slow and controlled release of medication over an extended period of time but without foreign-body awareness and retention problems of the prior art.